In recent years, a battery using a nonaqueous solvent such as a lithium ion secondary battery has been developed as a high energy density type secondary battery. The lithium ion secondary battery is excellent in the energy density and cycle characteristic as compared to a lead storage battery and a nickel hydrogen secondary battery, and is expected as a large storage battery used as a vehicle power supply for a hybrid automobile or an electronic automobile.
As the electrolyte of the lithium ion secondary battery, for example, a nonaqueous electrolyte such as ethylene carbonate, diethyl carbonate, or propylene carbonate is used from the viewpoint of a wide electrochemical window. There is a problem with the safety of these solvents because they are combustible. For this reason, if the nonaqueous solvent-based electrolyte can be replaced with an aqueous solution-based electrolyte, this problem can be solved thoroughly. In addition, the aqueous solution-based electrolyte is more inexpensive than the nonaqueous solvent-based electrolyte, and the manufacturing process does not need an inert atmosphere. Hence, when the nonaqueous solvent-based is replaced with the aqueous solution-based electrolyte, the cost is expected to be largely reduced.
However, there is a large problem with the use of the aqueous solution-based electrolyte in the lithium ion secondary battery. A theoretical decomposition voltage calculated based on the chemical equilibrium of water is 1.23 V. Hence, if a battery is formed based on a higher design voltage than the theoretical decomposition voltage, oxygen is generated in the positive electrode, and hydrogen is generated in the negative electrode.